This invention relates to fused tapered couplers, including those with and those without in-fiber Bragg gratings, the operating parameters of which are trimmed by precisely focused ultraviolet radiation or by means of ultraviolet radiation controlled by precision masks.
Management of high-capacity broadband channels in a dense wavelength-multiplexed optical network requires the use of low-loss filters that can add (multiplex) or drop (de-multiplex) signals at closely spaced wavelengths. A common means to realize this capability is to use a device that contains one or more fiber Bragg gratings since they can be designed to isolate adjacent channels, avoid cross talk, and have low insertion loss.
It has been known for many years that fused tapered fiber coupler technology can be used to multiplex signals and route channels provided the channels are not too closely spaced. A fused tapered coupler 13 consists of a pair of identical fibers that are fused along a segment of length and then drawn so as to taper down 14 to a small waist 15 and taper up 16 to the original size, as shown in FIG. 1. Light launched into one fiber core (for example, at port A in FIG. 1) couples into the modes of the fused and tapered composite fiber and spreads out to the cladding boundary in the waist 15 of the coupler where the cross section of the cores are reduced. The transverse distribution of light in the fused fiber cycles from one side to the other as the modes propagate along the device. The amount of light which appears in the exit ports, C and D in FIG. 1, is determined by the length of the device, the degree of fusion, the axial profile of the cross section, and the optical wavelength. If the taper is gradual enough, namely if it is adiabatic with regard to the evolution of the modal amplitudes, then the field may be regarded as a liner superposition of the lowest order modes of the fused fiber. These modes propagate at different velocities and consequently interfere. When they are in-phase, the light is concentrated in one-half of the fiber cross-section and when they are in anti-phase it switches to the opposite side. This occurs because the modes of the composite fiber, commonly referred to as supermodes to distinguish them from the modes of an individual single core fiber, are distributed over most of the fiber cross-section with maxima at the core centers, and are even, symmetric about the midplane of the waist cross section, and odd, asymmetric the midplane of the cross-section. The total phase difference, xcex94xcfx86, determines the amount of light that will be coupled from the input fiber at port A to the adjacent fiber at the pass port C and the drop port D.
The coupler is fabricated by drawing the length to the point wherein a desired fraction of the input light appears in the cross-coupled port, C. In the simplest instance, when the wavelength of the light is varied over a small range, the phase will change inversely with the wavelength, and the light will cycle cosinusoidally between the exit ports. The coupler acts as a filter, separating a desired wavelength through interference of the even and odd supermodes of the fused fiber.
As demand for capacity increases and channel spacing is reduced to less than 200 GHz, fused tapered couplers cannot be designed to realize the narrowband filtering capability. However, if an in-fiber Bragg grating (FBG) is incorporated into the waist of a coupler then the spectral characteristics are governed by the grating design rather than by the coupler. See, for example, Kashyap, R., Fiber Bragg Gratings, Academic Press, San Diego (1999), Section 6.7, pp. 276-284. Using fibers which are photosensitive, an FBG 18 can be formed in the waist 19 of the coupler 20 (FIG. 2) by ultraviolet light (UV) exposure using a phase mask or a pair of interfering beams. The grating will reflect a portion of each supermode if the wavelength falls within the grating stop-band. The stop-band of the even and odd modes will be very nearly the same; however, they are not identical because the Bragg wavelengths are different. The coupler can be designed so that the Bragg wavelengths of the two modes differ only by a very small fraction of the stop-band by choosing the degree of fusion and taper ratio at the waist. In this device, the filter characteristics are governed by the grating properties, namely, the length, strength and apodization, and by the phase of the reflected and transmitted supermodes.
The presence of the grating changes the phase. In reflection, namely for those wavelengths within the stop-band, the modes reflect before they reach the end of the grating and the phase of each depends on the grating properties. A reflected pulse of light will be delayed by a round-trip time equal to the transit time within the grating. In the case of a weak grating, the mode will appear to be reflected from the center of the grating. If the grating is made stronger, the effective length of the reflection point, as measured by the group delay, will decrease. Thus, with a very strong grating, the reflection will occur at a point quite close to its edge. In transmission, the phase will also depend on the strength of the grating because the average index of the fiber will be increased as a result of the ultraviolet (UV) exposure used to form the grating. The presence of the grating changes the interference of the modes in reflection and in transmission. It follows that the return loss, namely the amount of light that is reflected back into the input port A and thereby lost from the drop port D, may increase. In addition, the coupling ratio, or equivalently, the fraction of the light that is transmitted directly through to port B (leakage) rather than being cross-coupled into port C, will also be changed when the grating is formed.
Precise placement of the grating within the coupler waist, to minimize the return loss and leakage, is difficult because of the tolerances required: location of the grating center with a precision of several hundred microns may be required. And the characteristics of the coupler change as a consequence of the index of refraction changing, due to irradiation by UV light in the process of making the grating.
In X. Daxhelet and S. Lacroix, xe2x80x9cUV-Trimming of Fused Fiber Coupler Spectral Response: A Complete Modelxe2x80x9d, IEEE Photonics Technology Letters, Vol. 10, No. 9, pp. 1289-1291, a fused-fiber coupler, in which only the cores are photosensitive, is exposed to UV. This paper concludes that not much change could be expected in the phase difference between the two fundamental supermodes for the case where both core and cladding are photosensitive, as is true for FBG, fused tapered add and/or drop filters. In the case of a drop filter (having no add function), the trimming may be simplified because each end of the device can be exposed separately and independently. However, uniform exposure of the fused fiber, particularly if it has low photosensitivity or uses a doped photosensitive cladding, will not produce a large enough change in the differential phase of the supermodes, even when trimming a drop filter, except in those cases where a very small change will suffice. For an add/drop filter (multiplexer/demultiplexer), the need to trim both sides of the waist is even more acute.
Objects of the invention include trimming of a fused-taper coupler, with or without a fiber Bragg grating, in a manner to either increase or decrease the phase differential between the symmetric and asymmetric supermodes; compensation for changes produced by grating fabrication in fiber Bragg grating fused-taper couplers; compensation for changes produced by inaccurate location of the Bragg grating in fused-taper couplers; and fused-taper couplers, with or without a fiber Bragg grating, having optimized performance.
This invention is predicated on the discovery that trimming of a fused tapered coupler by UV exposure of a selected portion of the cross section of one or both end regions of the coupler, including regions of the waist (which do not contain the grating of a coupler employing an FBG) and the nearby tapered regions of the coupler, is an effective way to either increase or decrease the differential phase of the supermodes to improve the amount of coupling into the drop port, to reduce the loss of the reflected power into the input port, and to decrease the insertion loss.
According to the present invention, a fused tapered coupler multiplexer and/or demultiplexer is phase adjusted by exposing one or two selectively limited portions of the cross section of one or more lengths of the coupler to ultraviolet light, which is focused precisely, or controlled by a mask having one or two slits, laterally centered on the waist of the coupler, to achieve an appropriate differential phase by favoring a phase change for either the symmetric (even) or asymmetric (odd) supermode, making it possible to trim each section of the fused pair differently, or even separately, if necessary.
In accordance with the invention, the two sides of the waist of a fused tapered coupler are trimmed separately so as to selectively favor adjustment of the index of refraction (index, indices, or n, hereinafter) of either the symmetric or asymmetric supermode, and both sides of the waist of such a coupler may be trimmed so as to be able to substantially optimize both the add function and the drop function, with minimum insertion loss and reflection back to the upstream sources.
In accordance with the invention, exposing a section of the fiber coupler with one or two precise patterns of ultraviolet light, laterally centered on the cross-section, causes a large change in the differential phase, adjusted to favor trimming the index of either the symmetric (even) or asymmetric (odd) supermode. As a result, the differential or beat phase, xcex94xcfx86, can be selectively decreased significantly as well as increased, providing more flexibility in trimming both sides of the device for add and drop functions.
This invention provides the means to trim the device to obtain both add and drop multiplexing with one coupler, with or without an FBG. The mode selective precise trimming method of the invention is especially well-suited for couplers made with a large photosensitive-doped cladding. The use of such a fiber has a twofold advantage: it increases the grating reflectivity for a given change in the index, and also reduces out-of-band cladding mode coupling loss.
The invention may be practiced in couplers employing an FBG as well as couplers not having an FBG. In couplers employing an FBG, the invention allows precise trimming to compensate for index changes brought on by UV radiation when forming the grating, as well as trimming to compensate for positioning and other fabrication variations.
As used herein, the term xe2x80x9cend regionxe2x80x9d includes portions of the waist of a coupler, excluding any portion having an FBG formed therein, or portions of the tapered regions of a coupler in which significant changes can be made in the index of refraction by UV irradiation, or both portions.
Other objects, features and advantages of the present invention will become more apparent in the light of the following detailed description of exemplary embodiments thereof, as illustrated in the accompanying drawing.